A diaphragm-type carburetor is employed, as are most carburetors, to supply fuel to an internal combustion engine, particularly, to such an engine having a limited displacement. Diaphragm carburetors are generally used to supply fuel to two-cycle engines. These carburetors are equipped with a fuel pressure regulator that ensures fuel fed from a fuel pump is regulated at a fixed pressure, and then delivered to an air intake path. The fuel pressure regulator is typically equipped with a constant-pressure fuel chamber that stores fuel sent from the fuel pump. The constant-pressure or metering fuel chamber is generally separated from atmosphere by a diaphragm that adjusts the fuel pressure to a constant pressure. A control valve that is interlocked to the motion of the diaphragm opens and closes a fuel passageway through which fuel flows to the fuel chamber. Fuel from the fuel chamber is delivered to the air intake path via a main fuel path and an idle fuel path. The main fuel path leads to a main nozzle that is open to a venturi in the air intake path. The idle fuel path leads to slow and idle ports that open adjacent to a throttle valve in the air intake path.
Referring to FIG. 1, a prior art carburetor having a fuel supply and control circuit is shown. The carburetor 1 includes a body 2 with an air intake path 5 that extends horizontally, and covers 3 and 4 mounted on the top and bottom of the body 2. The intake path 5 has a venturi 6 and a throttle valve 7 mounted upstream of the venturi 6.
A fuel pump diaphragm 9 of a fuel pump 8 is sandwiched between the body 2 of the carburetor 1 and the top cover 3. Fuel in a fuel tank (not shown) passes from a fuel pipe 10 through an inlet valve 11, an inlet chamber 12, a pump chamber 13, an outlet valve 14, and an outlet chamber 15, and is fed, via a fuel path 17 to a metering or constant-pressure fuel chamber 20 of a fuel pressure regulator 18. A pulse pressure generated in an engine crankcase is introduced into a pulse chamber 16 which opposes a pump chamber 13 (both of which sandwich the fuel pump diaphragm 9), which causes the fuel to be sucked into the pump chamber 13, from which it is dispensed, all of which is generally known in the art.
A metering diaphragm 19 of a fuel pressure regulator 18 is sandwiched between the body 2 and the bottom cover 4 of the carburetor 1, and separates the fuel chamber 20 above from an air chamber 21 below. A lever 23, which is housed in the fuel chamber 20 and supported in free rotation by a pin 22, is biased by a spring 24 so one end 23a of the lever 23 contacts the center of the metering diaphragm 19. At the other end 23b, the lever 23 supports an inlet needle 25 of a fuel control valve 33 that opens and closes the fuel path 17. When the pressure drops in the fuel chamber 20 as fuel is fed from the chamber 20 into the air intake 5, the metering diaphragm 19 is biased upward, biasing the inlet needle 25 downward or away from the control valve 33 to open the control valve 33 and allow fuel to flow through the fuel path 17 into the fuel chamber 20. When the pressure rises in the fuel chamber 20 due to the flow of fuel into the chamber 20, the metering diaphragm 19 is biased downward, biasing the inlet needle 25 upward or toward the control valve 33 to close the control valve 33. In this manner, the fuel chamber 20 is always kept at a constant pressure.
The fuel from the fuel chamber 20 enters a nozzle chamber 27 via a main fuel path 26. The fuel is fed from the nozzle chamber 27 to the air intake path 5 through a main nozzle 28 that opens into the venturi 6 of the air intake path 5. The fuel from the fuel chamber 20 also enters a port chamber 30 via an idle fuel path 29. Depending on the position of the throttle valve 7, the fuel is fed from the port chamber 30 into the air intake path 5 through an idle port 31 or part throttle ports 32 adjacent to the throttle valve 7.
In conventional diaphragm carburetors, such as the prior art carburetor shown in FIG. 1, a main fuel jet is fixedly mounted within the carburetor body. Thus, it would be desirable to provide a carburetor with a simplified fuel supply and metering system that tends to reduce material and labor costs.